Warm-air furnace systems are well-known and generally recover useful sensible heat from system combustion product gases using at least one or more system elements which extract varying degrees of available heat. In some instances the well-known systems recover sensible heat from combustion product gases flowing through system combustor elements or in a system firepot element submerged in the system stream of warm-air. Also, such systems may utilize one or more heat-exchanger or heat-exchanger-like elements to obtain additional sensible heat from the system combustion product gases for transfer to the system stream of warm-air. Such systems, however, have not been known to advantageously extract available latent heat from the flow of system combustion product gases and utilize that heat to pre-heat the system airflow stream prior to the airflow stream being additionally heated by the system primary sensible heat source or sources.
For examples of known heating systems which do recover latent heat from system combustion product gases but which do not utilize that heat for airstream pre-heat purposes see U.S. Pat. No. 4,340,572 issued in the name of Ben-Shmuel et al., U.S. Pat. No. 4,799,941 issued in the name of Westermark, and U.S. Pat. No. 4,919,085 issued in the name of Kaisha.
Also, conventional warm-air furnace systems wherein latent heat is recovered from combustion product gases to increase system thermal efficiency are well-known but such systems do not advantageously utilize a direct contact condensing heat exchanger for that purpose. By way of example refer to the Amana "Air Command 90" furnace system.